Method of passivating an iron disulfide surface via encapsulation in a zinc sulfide matrix

ABSTRACT

A method for passivating the surface of crystalline iron disulfide (FeS 2 ) by encapsulating it within an epitaxial zinc sulfide (ZnS) matrix. Also disclosed is the related product comprising FeS 2  encapsulated by a ZnS matrix in which the sulfur atoms at the FeS 2  surfaces are passivated. Additionally disclosed is a photovoltaic (PV) device incorporating FeS 2  encapsulated by a ZnS matrix.

PRIORITY CLAIM

The present application is a divisional of U.S. application Ser. No. 14/661,165, filed on Mar. 18, 2015, which claims the benefit of U.S. Provisional Application No. 61/954,703, filed on Mar. 18, 2014 by Jesse A. Frantz et al., entitled “Method of Passivating an Iron Disulfide Surface via Encapsulation in Zinc Sulfide.” The entire contents of these applications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to passivating the surface of crystalline iron disulfide (FeS₂) by encapsulating it in crystalline zinc sulfide (ZnS). It also relates to FeS₂ encapsulated by ZnS in which the sulfur atoms at the FeS₂ surfaces are passivated. Additionally, this invention relates to a photovoltaic (PV) device incorporating FeS₂ encapsulated by ZnS.

Description of the Prior Art

Iron disulfide has great promise as an Earth-abundant material for PV applications. In a recent survey of 23 known semiconductor systems with potential as PV absorbers, FeS₂ ranked highest in potential annual electricity production based on known reserves and had the lowest extraction cost. Wadia et al., “Materials availability expands the opportunity for large-scale photovoltaics deployment,” Environ. Sci. Technol., 43, pp. 2072-2077 (2009). Its bandgap is 0.95 eV, high enough to result in a potential solar to electricity conversion efficiency similar to that of Si, but unlike Si, it has an exceptionally high absorption coefficient of α=6×10⁵ cm⁻¹ resulting in a required thickness of <40 nm for >90% absorption (Ennaoui et al., “Iron disulfide for solar energy conversion,” Sol. Energ. Mat. Sol. Cells, 29, pp. 289-370 (1993)) compared to typical thicknesses >100 μm for Si.

Despite these apparent advantages, FeS₂ PV devices have not yet lived up to their potential. Efficiency has been limited to approximately 3% due to open circuit voltage (V_(OC)) of <200 mV, only ˜20% of the bandgap. Ennaoui et al., “Iron disulfide for solar energy conversion,” Sol. Energ. Mat. Sol. Cells, 29, pp. 289-370 (1993) and Wilcoxon et al., “Strong quantum confinement effects in semiconductors: FeS₂ nanoclusters,” Sol. State Comm., 98, pp. 581-585 (1996). These limits have been shown to be the direct result of surface termination of FeS₂ crystals. Bulk FeS₂ crystallizes in the cubic pyrite structure, and its sulfur atoms are paired in an S—S bond (S₂ ²⁻). Crystal surfaces, however, are typically terminated by S monomers (S¹⁻) that may convert to S²⁻ through a redox reaction. Bi et al., “Air stable, photosensitive, phase pure iron pyrite nanocrystal thin films for photovoltaic application,” Nano Lett., 11, pp. 4953-4957 (2011) and Zhang et al., “Effect of surface stoichiometry on the band gap of the pyrite FeS₂(100) surface,” Phys. Rev. B, 85, art. 085314 (2012). The resulting surface states exhibit a high density of defects within the FeS₂ bandgap and shows properties similar to the iron monosulfide phase, with a bandgap of approximately 0.3 eV. In PV devices, these surface states at films' surfaces and grain boundaries lead to high dark current and low V_(OC).

BRIEF SUMMARY OF THE INVENTION

The aforementioned problems are overcome in the present invention which provides a method of passivating the surface of crystalline iron disulfide (FeS₂) by encapsulating it in crystalline zinc sulfide (ZnS), a product comprising FeS₂ encapsulated by ZnS in which the sulfur atoms at the FeS₂ surfaces are passivated, and a photovoltaic (PV) device incorporating FeS₂ encapsulated by ZnS.

To move beyond its current performance bottleneck, FeS₂ requires passivation of its surface states. The present invention provides a method of passivating surface defects in FeS₂ by encapsulating it in ZnS. A density-functional theory (DFT) study indicates that ZnS can create a defect-free interface with FeS₂. Experimental results indicate that surface defects in polycrystalline FeS₂ films are indeed passivated by encapsulation in ZnS.

The present invention has many advantages. It results in passivation of surface states in crystalline FeS₂, which is a feature that has not been shown with any other encapsulant. Using the method of the present invention, the FeS₂/ZnS interface can be made free of mid-gap states that are typically associated with S monomers at the FeS₂ surface. Also, this invention can be incorporated into a rigid or flexible PV device, making it possible to build efficient solar cells from an Earth-abundant material.

These and other features and advantages of the invention, as well as the invention itself, will become better understood by reference to the following detailed description, appended claims, and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a polycrystalline FeS₂ film encapsulated by coating it with a ZnS capping layer.

FIG. 2 shows XPS results comparing the S 2P peak. FIG. 2a is for FeS₂ encapsulated by a ZnS capping layer. FIG. 2b is for FeS₂ encapsulated by a ZnO capping layer. FIG. 2c is for FeS₂ encapsulated by a SiO₂ capping layer.

FIG. 3 shows the atomic structure of an FeS₂ nanocrystal embedded in a ZnS matrix, based on DFT results.

FIG. 4 shows FeS₂ crystallites encapsulated within a ZnS matrix. In FIG. 4a , the substrate is a rigid material. In FIG. 4b , the substrate is a flexible material.

FIG. 5 shows FeS₂ crystallites encapsulated within a ZnS matrix being employed in a PV device.

DETAILED DESCRIPTION OF THE INVENTION

In one embodiment, FeS₂ is sputtered at room temperature from a single target in a partial pressure (1×10⁻⁵ T) of sulfur onto a glass substrate. The film was ˜200 nm thick and polycrystalline. It exhibited the expected cubic pyrite crystal structure as indicated by X-ray diffractometry. The sample was transferred to an evaporation chamber without removal to atmosphere, and a 40 nm thick layer of epitaxial ZnS was deposited by thermal evaporation. A sketch of this sample is shown in FIG. 1. Transferring the sample between deposition chambers under vacuum, or without removal to air, avoids oxidation and contamination of the FeS₂ surface. In other embodiments, high substrate temperature deposition of FeS₂ may be carried out by sputtering from a multi-component target to a high temperature substrate (e.g., Ts=400° C.), and sulfurdizing under flowing H₂S (e.g., at 500° C., for 5 hours). All vacuum-deposited fabrication is preferred according to some aspects of the invention.

Initial X-ray photoelectron spectroscopy (XPS) results for this sample were obtained and compared to results for bare FeS₂ and films with ZnO and SiO₂ encapsulation layers. The encapsulation layers were removed in steps inside an ultra-high vacuum chamber with an ion beam, and XPS scans were carried out after each removal step. The results, shown in FIG. 2, compare the S 2p doublet of a bare film to those with capping layers. In each case a combination of S 2p doublets associated with both the bulk states and surface defects is present. The peak with lowest binding energy (near ˜161 eV) is the S 2p_(3/2) component of the doublet and is associated with these surface defects. For both ZnO and SiO₂, this peak is stronger than the peak associated with the bulk states, indicating a larger concentration of surface defects, presumably S²⁻. For the ZnS-capped sample, however, the defect peak is smaller relative to the bulk peak, indicating that the surface defects have been partially passivated. This is the first demonstration of passivation of FeS₂ surface defects by a ZnS capping layer.

To obtain an atomic scale understanding of the bonding between FeS₂ and ZnS, DFT calculations were carried out. The FeS₂ and ZnS have a nearly perfect lattice match, with lattice spacings of 5.417 Å and 5.411 Å, respectively. Because of this the two materials can form a nearly defect-free interface. An illustration of an FeS₂ nanocrystal encapsulated in ZnS, based on DFT, is shown in FIG. 3.

Several other embodiments of the invention are shown in FIG. 4. In these cases, FeS₂ crystallites are encapsulated within a ZnS matrix. The FeS₂ crystallites may vary in size from 1 nm to 10 μm, and the ZnS separating FeS₂ crystallites is at least one monolayer thick. FIG. 4a shows an embodiment in which the substrate is a rigid material such as rigid glass or a semiconductor wafer; and FIG. 4b shows an embodiment in which the substrate is a flexible material such as polymer, flexible glass, or metal foil. In the latter case, the FeS₂ and ZnS matrix constitute a film that may flex along with the substrate.

In another embodiment, the film comprising FeS₂ crystallites encapsulated within a ZnS matrix is employed as the absorber in a PV device. One example of a suitable device architecture is shown in FIG. 5. In this example, the device comprises a substrate, a conductive bottom contact, the FeS₂ crystallites encapsulated within a ZnS matrix, a transparent p-type layer, a transparent conductive to serve as the top contact, and a metal grid that aids efficient charge collection. FeS₂ is typically an n-type semiconductor, so in this architecture, the transparent p-type layer is used in conjunction with the FeS₂/ZnS layer to form a p-n junction. Any other suitable PV device architecture, such as a Schottky junction device, could be used.

The FeS₂ crystallite size may vary from 1 nm to 10 cm. Individual crystallites may be in contact, as is the case in polycrystalline bulk samples or thin films, or crystallites may be separated with each entirely encapsulated in ZnS. The FeS₂ may be a natural or synthetic bulk sample.

The FeS₂ may be a film deposited by any suitable deposition technique. This technique may be any physical vapor, chemical vapor deposition, atomic layer deposition, or other suitable deposition process.

The ZnS may be a film deposited by any suitable deposition technique. This technique may be any physical vapor, chemical vapor deposition, or other suitable deposition process. The S content in FeS₂ could vary by up to ±20% from stoichiometry.

The Fe in FeS₂ could be partially substituted by Si with a ratio of up to 50%, i.e. Fe_(1-x)Si_(x)S₂ where x<0.5. The Zn in ZnS could be partially substituted by another metal including Ni, Mn, Cu, Ag, or Pb with a ratio of up to 50%. The S in ZnS could be partially substituted by Se or O with a ratio of up to 50%.

The above descriptions are those of the preferred embodiments of the invention. Various modifications and variations are possible in light of the above teachings without departing from the spirit and broader aspects of the invention. It is therefore to be understood that the claimed invention may be practiced otherwise than as specifically described. Any references to claim elements in the singular, for example, using the articles “a,” “an,” “the,” or “said,” is not to be construed as limiting the element to the singular. 

What is claimed as new and desired to be protected by Letters Patent of the United States is:
 1. A method for passivating iron disulfide crystallites, comprising: forming iron disulfide crystallites comprising crystal surfaces; and encapsulating the iron disulfide crystallites within an epitaxial zinc sulfide matrix; wherein the epitaxial zinc sulfide matrix passivates sulfur atoms present on the crystal surfaces of the iron disulfide crystallites, thereby reducing surface defects as compared to iron disulfide crystallites not encapsulated by an epitaxial zinc sulfide matrix.
 2. The method of claim 1, further comprising placing the zinc sulfide matrix comprising encapsulated iron disulfide crystallites on a substrate.
 3. The method of claim 2, wherein the substrate comprises a rigid material.
 4. The method of claim 2, wherein the substrate comprises a flexible material.
 5. The method of claim 4, wherein the epitaxial zinc sulfide matrix and encapsulated iron disulfide crystallites form a film that flexes along with the flexible substrate.
 6. The method of claim 1, wherein the iron disulfide crystallites range in size from 1 nm to 10 μm.
 7. The method of claim 1, wherein a layer of the epitaxial zinc sulfide matrix at least one monolayer thick separates the iron disulfide crystallites.
 8. The method of claim 1, wherein the surface defects in the iron disulfide crystallites are assessed by comparing an X-ray photoelectron spectroscopy scan of S 2p doublets associated with surface defects with an X-ray photoelectric spectroscopy scan of S 2p doublets associated with the bulk state.
 9. The method of claim 1, wherein the crystal surfaces of the iron disulfide crystallites and the epitaxial zinc sulfide matrix form a lattice match.
 10. The method of claim 1, wherein the epitaxial zinc sulfide matrix comprises crystal surfaces having a lattice constant of about 5.411 Å.
 11. The method of claim 1, wherein the crystal surfaces of the iron disulfide crystallites have a lattice constant of about 5.417 Å.
 12. The method of claim 1, wherein the epitaxial zinc sulfide matrix is deposited by physical vapor deposition.
 13. The method of claim 1, wherein the epitaxial zinc sulfide matrix is deposited by chemical vapor deposition.
 14. The method of claim 13, wherein the chemical vapor deposition is atomic layer deposition.
 15. The method of claim 1, wherein the iron disulfide crystallites are formed by physical vapor deposition.
 16. The method of claim 1, wherein the iron disulfide crystallites are formed by chemical vapor deposition.
 17. The method of claim 16, wherein the chemical vapor deposition is atomic layer deposition.
 18. The method of claim 1, further comprising incorporating the passivated iron disulfide crystallites into a photovoltaic device. 